Signalling system

ABSTRACT

A battery signalling system is provided which can be used to monitor and/or control a battery (1) having a number of series connected battery cells (C i ) . When used to monitor the battery cells, the battery signalling system can comprise a central battery monitoring system (3) for monitoring the industrial battery (1) as a whole, a number of cell monitoring devices (CM i ) for monitoring one or more battery cells (C i ) and a communication link (9) for connecting the cell monitoring devices (CM i ) in series in a daisy chain configuration to the central battery monitoring system (3). In operation, the central battery monitoring system (3) can poll each of the cell monitoring devices ( Cmi ) in turn and analyze the data received from a polled cell monitoring device (CM i ) to detect malfunctions and/or underperforming cells.

This application is a CIP patent application of InternationalApplication PCT/GB98/00170 filed Jan. 20, 1998.

FIELD OF THE INVENTION

The present invention relates to a signalling system. The invention isapplicable for use in a system for monitoring and/or controlling thecells of an industrial battery.

BACKGROUND OF THE INVENTION

Industrial batteries comprise a number of rechargeable battery cellswhich can be electrically connected in various series andseries-parallel combinations to provide a rechargeable battery having adesired output voltage. To recharge the battery, a current is passedthrough the cells in the opposite direction of current flow when thecells are working. There are many different types of battery cellsavailable, but those most commonly used in industrial applications arelead acid battery cells, each of which provides 2 volts, andnickel-cadmium (Nicad) battery cells, each of which provides 1.2 volts.

The batteries are usually used as a back-up power supply for importantsystems in large industrial plants, such as off-shore oil rigs, powerstations and the like. Since the batteries are provided as back-up inthe event of a fault with the main generators, they must be constantlymonitored and maintained so that they can provide power to the importantsystems for a preset minimum amount of time.

Many battery monitoring systems have been proposed which monitor thebattery as a whole and provide an indication of the battery voltage.However, only a few systems have been proposed which can also monitorthe individual cells which make up the battery. These systems use anumber of monitoring devices, some of which are powered by the batterycell or cells which they monitor and send status information indicativeof the cell voltage back to a central battery monitory system whichmonitors the battery as a whole.

However, since the cells are connected in series and since each cellmonitoring device is powered by the cell which it is monitoring, theground or reference voltage of each cell monitoring device is different.For example, in an industrial battery which has sixty lead acid cellsconnected in series, the negative terminal, i.e. the ground, of thefifth cell will be at a potential of approximately 8 volts and thepositive terminal will be at a potential of approximately 10 volts,whereas the negative terminal of the seventh cell will be at a potetialof approximately 12 volts and the positive terminal will be at apotential of approximately 14 volts. This has lead to the commonmisconception in the art that the cell monitoring devices have to beelectrically isolated from each other and from the central batterymonitoring system.

In one known cell monitoring system, each cell is independently linkedto its own electrically isolated input at the central monitoring system.The problem with this system is that a large number of connectors areneeded to link the individual cell monitoring devices to the centralmonitoring system. Consequently, in practice, it is seldom used forpermanent real-time monitoring of the battery cells.

In another known cell monitoring system, each cell monitoring device isserially linked to its neighbours in a daisy-chain configuration, eitherby using optical links between the monitoring devices or by usingtransformers which have no DC path. The problem with this system is thatto operate, each of the cell monitoring devices requires either anelectrical to optical and an optical to electrical converter or amodulator and a demodulator, which makes them relatively expensive andinefficient since this additional circuitry requires more power from thecell.

There is therefore a need to provide a simple cell monitoring devicewhich can monitor and report on the status of the cells of the battery,but which consumes minimal power from the cell which it is monitoring.

As mentioned above, existing battery monitoring systems monitor thebattery and provide an indication of the battery voltage. However,battery voltage is not an indication of the capacity of the battery,i.e. the ability of the battery to provide energy. There is thereforealso a need to provide a battery monitoring system which can give theuser a fairly accurate estimate of how much load he can place on abattery and over what period of time.

SUMMARY OF THE INVENTION

The inventor has realised that it is possible to overcome the problem ofhaving the cell monitoring devices operating at different voltages usingsimple electronic components and that therefore, there is no need forelectrical isolation between the individual cell monitoring devices andthe central monitoring system.

According to a first aspect, the present invention provides a signallingsystem for use with a plurality of series connected battery cells,comprising: a plurality of cell signalling devices, each to be poweredby a respective one or more of the plurality of battery cells; and acommunication link connecting the plurality of cell signalling devicesin series; wherein each cell signalling device comprises a level shiftcircuit which is operable to receive signals transmitted from anadjacent cell signalling device to shift the level of the receivedsignal and to output the level shifted signal for transmission to thecommunication link. By providing a level shift circuit in each cellsignalling device, the cell signalling devices can be linked together ina communication link without the need for electrical isolation betweenthe signalling devices.

The signalling system can be used as part of a battery monitoring and/orcontrol system which is used to monitor and/or control the seriesconnected battery cells. By providing the level shift circuit in eachcell signalling device, the signalling system obviates the need forelectrical isolation between individual cell signalling devices.Consequently, the communication link can be a simple one-wirecommunication bus.

Preferably each of the cell signalling devices is able to receivecommunications from and transmit communications to the communicationlink so that they can communicate with, for example, the batterymonitoring and/or control system. In which case, each cell signallingdevice can comprise two DC level shift circuits, one for increasing thelevel of the received signals for transmission to a cell signallingdevice having a higher ground potential than that of the receiving cellsignalling device, and one for reducing the level of the receivedsignals for transmission to a cell signalling device which has a lowerground potential than that of the receiving cell signalling device.

Each level shift circuit can comprise a simple electronic device, suchas a comparator, which consumes a relatively small amount of power fromthe battery cell which powers the cell signalling device.

The first aspect of the present invention also provides a cellsignalling device for use in the above defined signalling system,comprising: a power input terminal connectable to the cell or cellswhich is or are to power the cell signalling device; and at least one DClevel shift circuit for receiving signals from an adjacent cellsignalling device, for shifting the level of the received signal, andfor outputting the level shifted signal for transmission to thecommunication link.

The first aspect of the present invention also provides a signalling kitcomprising a plurality of the cell signalling devices defined above. Thekit may also comprise the communication link for connecting the cellsignalling devices in series.

The first aspect of the present invention also provides a signallingmethod using a plurality of series connected battery cells, comprisingthe steps of: providing a plurality of cell signalling devices andpowering them with a respective one or more of the plurality of batterycells; providing a communication link which connects the plurality ofcell signalling devices in series; receiving signals transmitted from anadjacent cell signalling device; shifting the level of the receivedsignals; and outputting the level shifted signals to the communicationlink.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described, by way of example only,with reference to the accompanying drawings, in which:

FIG. 1 schematically shows a battery comprising a number of batterycells connected in series, a central battery monitoring system formonitoring the condition of the battery as a whole and individual cellmonitoring devices for monitoring the cells of the battery;

FIG. 2 is a schematic diagram showing more detail of the central batterymonitoring system shown in FIG. 1;

FIG. 3 is a schematic diagram of one of the cell monitoring devicesshown in FIG. 1;

FIG. 4 is a plot showing the battery-cell voltage distribution;

FIG. 5a is a circuit diagram of a first comparator forming part of thecell monitoring device shown in FIG. 3;

FIG. 5b is a circuit diagram of a second comparator forming part of thecell monitoring device shown in FIG. 3;

FIG. 5c is a schematic representation showing part of the battery-cellstaircase voltage distribution and example data pulses which are appliedto the input of the comparators shown in FIGS. 5a and 5b;

FIG. 6 is a schematic diagram of one of the cell monitoring devicesaccording to a second embodiment of the present invention;

FIG. 7 schematically illustrates the way in which signals are passedbetween adjacent cell signalling devices in the second embodiment of theinvention;

FIG. 8 schematically illustrates the form of a cell monitoring deviceaccording to a third embodiment and the way in which it is connected toneighbouring cell monitoring devices;

FIG. 9 schematically illustrates an alternative way in which signals canbe transmitted between adjacent cell signalling devices;

FIG. 10 is a schematic diagram of a battery cell monitoring device foruse in a battery monitoring system according to a second embodiment ofthe present invention;

FIG. 11 schematically shows a battery comprising a number of batterycells connected in series, a central battery control system forcontrolling the battery as a whole and individual battery cellcontrollers for controlling the cells of the battery;

FIG. 12 is a schematic diagram of one of the battery cell controldevices shown in FIG. 11;

FIG. 13 is a schematic diagram of a battery cell monitoring and controldevice for use in a battery monitoring and control system embodying thepresent invention;

FIG. 14 is a schematic representation of an industrial battery in whichthe cells of the battery are connected in a series-parallelconfiguration; and

FIG. 15 is a schematic diagram of a system for monitoring a plurality ofindustrial batteries.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

A first embodiment of the present invention will now be described withreference to FIGS. 1 to 5. FIG. 1 schematically shows an industrialbattery, generally indicated by reference numeral 1, comprising a numberof lead acid battery cells C₁, C₂, C₃ . . . C_(n) connected so that thenegative terminal C_(i) ⁻ of cell C_(i) is connected to the positiveterminal C_(i-1) ⁺ of preceding cell C_(i-1) and the positive terminalC_(i) ⁺ of cell C_(i) is connected to the negative terminal C_(i+1) ⁻ ofthe succeeding cell C_(i+1), whereby the negative terminal C_(i+1) ⁻ ofthe first cell C₁ is the negative terminal of the battery and thepositive terminal C_(n) ⁺ of the last cell C_(n) is the positiveterminal of the battery. Since the battery cells are lead acid, theyeach provide approximately 2 volts and the voltage of the battery as awhole will be approximately 2n volts. For industrial applications avoltage of 120 volts is often required. Therefore, 60 series connectedlead acid or 100 series connected Nicad battery cells would be required.Sometimes, each cell in the series connection is connected in parallelwith one or more similar cells, so as to provide redundancy, so that thebattery will not fail if a single cell fails.

FIG. 1 also shows a central battery monitoring system 3 which is poweredby the battery 1 via connectors 4 and 6, which connect the centralbattery monitoring system 3 to the negative terminal C₁ ⁻ and thepositive terminal C_(n) ⁺ of the battery 1, respectively. The batterymonitoring system 3 monitors the status of the industrial battery 1 as awhole, based on charging and discharging characteristics of the battery(determined by monitoring the battery voltage from connectors 4 and 6and the current being drawn from or supplied to the battery 1, which issensed by current sensor 8, whilst the battery is being charged andsubsequently discharged), the ambient temperature (input fromtemperature sensor 5) and on information relating to the efficiencycharacteristics of the battery cells (provided by the battery cellmanufacturer). The monitoring results can be stored in the centralbattery monitoring system 3 or they can be transmitted to a remote user(not shown) via the telephone line 7.

Each of the battery cells C_(i), shown in FIG. 1, also has a batterycell monitoring device CM_(i) mounted on top of the cell between itspositive and negative terminals Ci+and C_(i) ⁻ respectively, whichmonitors the status of the cell C_(i). Each cell monitoring deviceCM_(i) is powered by the cell C_(i) which it monitors and communicateswith the central battery monitoring system 3 via a simple one-wirecommunication link 9. The communication link 9 links the cell monitoringdevices CM_(i). in series in a daisy chain configuration to the centralbattery monitoring system 3, so that communications from the centralbattery monitoring system 3 to the cell monitoring devices CM_(i) passfrom left to right along the communication link 9 and communicationsfrom the cell monitoring devices CM_(i) to the central batterymonitoring system 3 pass from right to left along the communication link9. Each cell monitoring device CM_(i) has its own cell identification oraddress, which, in this embodiment, is set in advance using DIP-switchesmounted in the device. This allows communications from the centralbattery monitoring system 3 to be directed to a specific cell monitoringdevice and allows the central battery monitoring system 3 to be able toidentify the source of received communications.

The battery monitoring system shown in FIG. 1 operates in two modes. Inthe first mode, the central battery monitoring system 3 monitors thecondition of the industrial battery 1 as a whole and polls each of thecell monitoring devices CM_(i) in turn. During this mode, each of thecell monitoring devices CM_(i) listens to communications from thecentral battery monitoring system 3 on the communication link 9 andresponds when it identifies a communication directed to it. When polled,each cell monitoring device CM_(i) performs a number of tests on thecorresponding battery cell C_(i) and returns the results of the testsback to the central battery monitoring system 3 via the communicationlink 9.

In the second mode of operation, the central battery monitoring system 3listens for communications on the communication link 9 from the cellmonitoring devices CM_(i) indicating that there is a faulty conditionwith one of the battery cells C_(i). In this second mode of operation,each cell monitoring device CM_(i) continuously monitors thecorresponding battery cell C_(i) and, upon detection of a faultycondition, checks that the communication link 9 is free and then sendsan appropriate message back to the central battery monitoring system 3via the communication link 9.

FIG. 2 is a schematic diagram of the central battery monitoring system 3shown in FIG. 1. As shown, the central battery monitoring system 3comprises a CPU 11 for controlling the operation of the central batterymonitoring system 3. The CPU 11 is connected, via data bus 12, to a mainmemory 13 where data from the input sensors is stored and where testprograms are executed, to a display 15 which displays the battery'scurrent status and to a mass storage unit 17 for storing the sensor dataand the results of the battery tests. The mass storage unit 17 can befixed within the central battery monitoring system 3, but is preferablya floppy disk or a PCMIA memory card which can be withdrawn and inputinto an operator's personal computer for analysis. An operator can alsoretrieve the stored data and results and control the set up andinitialisation of the central battery monitoring system 3 via the RS-232serial interface 18. As mentioned above, instead of storing the testresults in the mass storage unit 17, they can be transmitted via a modem21 and telephone line 7 to a remote computer system (not shown) fordisplay and/or analysis.

The central battery monitoring system measures the total batterycapacity in Amp-hours (Ahr) or Watt-hours (Whr), the actual or remainingbattery capacity as a percentage of the total battery capacity and theinternal resistance of the battery 1 as a whole. The cental batterymonitoring system 3 can also measure the internal resistance of theindividual cells from the data received from the individual cellmonitoring devices CM_(i) received via the communication link 9 and thecommunication circuit 19.

In order to be able to measure the total battery capacity, i.e. themaximum amount of charge which can be stored in the battery, and theactual or remaining battery capacity at a given time point as apercentage of the total battery capacity, the central battery monitoringsystem 3 monitors how much charge is fed into the battery and how muchcharge is drawn from the battery. Unfortunately, since the charging anddischarging characteristics of the battery are not one hundred percentefficient. Therefore, the estimated capacity derived by monitoring thecharge alone is not very accurate. In fact, various factors affect theamount of charge which is input to or drawn from a battery duringcharging/discharging, including the ambient temperature, the magnitudeof the charging/discharging current, the algorithm used for chargingetc. Fortunately, many of these characteristics are known to the batterymanufacturer and, in this embodiment the specific characteristics of thebattery 1 are programmed into the central battery monitoring system 3.With this information, it is possible to determine more accurately howmuch charge has been stored in or withdrawn from the battery 1.

For example, if the battery 1 is charged with a charging current of 10amps over a period of two hours at an ambient temperature of 20° C., andit is known that the efficiency characteristic of the battery is 95% forsuch a level of charging current and for that ambient temperature, thenthe total charge supplied to the battery is 19 Ahr. In the general case,for a current (I(t)) drawn from or supplied to the battery, the capacity(CP) added to or removed from (depending on whether the current isnegative or positive) the battery from time t₀ to time t₁ is given by:##EQU1## where k_(i) are the efficiency characteristics for the battery1 for the sensed conditions and where I(t) is negative when the currentis being drawn from the battery 1.

In order to determine the initial total battery capacity (TCP), thebattery 1 is initially fully charged by charging the battery for a longperiod of time using a small charging current. Then the battery 1 isdischarged through a load (not shown) until the battery voltage dropsbelow an end of discharge voltage limit (EODV) which is specified by thebattery manufacturer. During this discharging period, the centralbattery monitoring system 3 monitors the discharge current via currentsensor 8, and once the EODV limit is reached, it calculates the capacity(in Amp-hours) which has been removed from the battery using equation 1above, with to being the time that the discharge is initiated and timet, is the time that the EODV limit is reached. This capacity representsthe total battery capacity (TCP).

In this embodiment, the central battery monitoring system 3 periodicallydetermines the remaining battery capacity (RCP) as a percentage of thetotal battery capacity (TCP) by monitoring the amount of current whichis drawn from and/or supplied to the battery 1 since the last time theremaining battery capacity was determined and then by using thefollowing equation: ##EQU2## Where CP[t₀,t₁ ] is calculated usingequation 1 above. The initial estimate for the remaining batterycapacity is set equal to the total working capacity of the battery afterthe battery has been fully charged.

To determine the internal resistance of the battery as a whole, thebattery is connected to two different loads and the central batterymonitoring system 3 monitors the current through the loads from which itdetermines the internal resistance of the whole battery.

As mentioned above, in addition to determining the total batterycapacity, the remaining battery capacity and the battery internalresistance, the central battery monitoring system 3 also monitors datareceived from the cell monitoring devices CM_(i) via the communicationcircuit 19 and the communication link 9. If there is a fault with one ofthe battery cells C_(i) or if there is some other faulty condition, theCPU 11 can trigger a local alarm 23 to alert a technician that there isa fault with the battery 1 or with one or more of the battery cellsC_(i). In this embodiment, the conditions which define a fault and theirthresholds are user definable and set in advance.

Although the central battery monitoring system 3 continuously monitorsthe battery 1, the sensor data and the other battery data, i.e. theremaining battery capacity etc., are only stored periodically in themass storage unit 17 in order to save storage space. The period isspecified in advance by the user and in this embodiment is set at tenseconds. Furthermore, when the samples are stored, they are time anddate stamped so that the battery charging and discharging behaviour canbe monitored and used to detect the cause of an eventual batteryfailure. In this embodiment, the data which is to be stored is alsofiltered in order to try to identify and highlight important events, andthe filtered data is also stored in the mass storage unit 17. Whatcounts as an important event is user definable, but can be, forinstance, a temperature increase of 2° C. or a change in remainingbattery capacity of greater than 1% of the total battery capacity.

As mentioned above, the status data of the battery, i.e. the batteryvoltage, the discharge/charge current, the battery temperature and theremaining and total battery capacities, are displayed on display 15. Forsimplicity, since the display 15 does not need to be continuouslyupdated, it is only updated using the samples of the status data whichare to be stored in mass storage unit 17. Therefore, in this embodiment,the display 15 is updated every ten seconds.

In this embodiment, the central battery monitoring system 3 is also usedto control the battery charger (not shown) which is used to charge thebattery 1. In particular, the central battery monitoring system 3monitors the charging current, the remaining battery capacity, theambient temperature etc. and controls the operation of the charger (notshown) so that the battery charging is in accordance with the specificcharging procedures recommended by the battery manufacturer for thebattery 1.

Since the total battery capacity also decreases with time (due toageing), the central battery monitoring system 3 is programmed toperform regular (for example daily or monthly) automated measurements ofthe total battery capacity and the battery internal resistance using theprocedures outlined above. This allows the central battery monitoringsystem 3 to be able to build up a picture of the battery lifecharacteristics and to be able to predict the battery end of life andthe early detection of faulty conditions.

FIG. 3 is a schematic diagram showing, in more detail, one of the cellmonitoring devices CM_(i). As shown, cell monitoring device CM_(i)comprises a microcontroller 31 for controlling the operation of the cellmonitoring device CM_(i) and for analysing sensor data received fromvoltage interconnection sensor 33, cell voltage sensor 35, temperaturesensor 37 and electrolyte level/PH sensor 39.

The voltage interconnection sensor 33 measures the voltage drop betweenthe cell being monitored and its neighbouring cells, by measuring thepotential difference between each terminal of the cell C_(i) and therespective terminal connections which connects cell C_(i) with itsneighbouring cells. Ideally, there should be no voltage drop betweeneach terminal and the corresponding terminal connection. However, due tochemical deposits accumulating at the cell terminals with time, orbecause of cell malfunction, a difference in potential between the cellterminals and the corresponding connectors sometimes exists, indicatingthat there is a fault, either with the battery cell C_(i) or with theinterconnection with a neighbouring cell.

The cell voltage sensor 35 is provided for sensing the potentialdifference between the positive terminal C_(i) ⁺ and the negativeterminal C_(i) ⁻ of the cell C_(i) which it is monitoring. Thetemperature sensor 37 senses the cell temperature locally at the cellC_(i). By monitoring the local temperature at each cell C_(i), it ispossible to identify quickly faulty cells or cells which are notoperating efficiently. The electrolyte level/PH sensor senses theelectrolyte level and/or the electrolyte PH of the battery cell C_(i)which it is monitoring.

The microcontroller 31 analyses the data input from the sensors andmonitors for faulty conditions and reports to the central batterymonitoring system 3 via the communication link 9. Since themicrocontroller 31 processes digital data, and since the signalsreceived from the sensors and the messages received from the batterymonitoring system 3 are analogue signals, the microcontroller 31 has abuilt-in analogue to digital convertor (not shown) so that it canconvert the sensor data and the received messages into correspondingdigital signals.

Since the cell monitoring devices are connected in series by thecommunication link 9, each cell monitoring device CM_(i) will eitherreceive communications originating from the central battery monitoringsystem 3, from the left hand side of the communication link 9 fortransmission to the next cell monitoring device CM_(i+1), or they willreceive communications from cell monitoring device CM_(i+1) from theright hand side of the communication link 9 for transmission back to thecentral battery monitoring system 3. In order to compensate for thedifference in reference voltages between each of the cell monitoringdevices CM_(i), each cell monitoring device CM_(i) has an up-link 41 fortransmitting data received from cell monitoring device CM_(i-1) to cellmonitoring device CM_(i+1), and a down-link 43 for transmitting datareceived from cell monitoring device CM_(i+1) to cell monitoring deviceCM_(i-1).

The up-link 41 has a transceiver 45 for increasing the reference voltageof the data signal so that it can be received by the next cellmonitoring device CM_(i+1), while the down-link 43 has a transceiver 47which reduces the reference voltage of the received data so that it canbe received by the cell monitoring device CM_(i-1). The up-link 41 andthe down-link 43 are connected to the one wire communication link 9 viaswitches 49 and 51 which are controlled by microcontroller 31, asrepresented by arrows 52. The way in which the microcontroller 31controls the position of the switches 49 and 51 for the above describedtwo modes of operation will be apparent to those skilled in the art andwill not be described here. The microcontroller 31 is connected to theup-link 41 by connection 53 so that it can listen for communicationssent from the central battery monitoring system 3 which are directed toit. Similarly, the microcontroller 31 is connected to the down-link 43by connection 55 so that the microcontroller 31 can send messages backto the central battery monitoring system 3, either upon being polled orupon detection of a fault.

In order to power the cell monitoring device CM_(i), the positiveterminal C_(i) ⁺ and the negative terminal C_(i) ⁻ of cell C_(i) areconnected to the input of a DC to DC convertor 57, which generates,relative to the ground or reference voltage V_(REF) ¹ of cell C_(i)(which equals the voltage potential of the negative terminal C_(i) ⁻ ofcell C_(i)) the voltages V_(REF) ^(i) ±5V, which are used to power themicrocontroller 31 and the transceivers 45 and 47.

FIG. 4 shows the voltage characteristic of the industrial batteryshowing each cell's terminal potential versus the cell's position in theseries. As shown in FIG. 4, this voltage characteristic has a staircaseshape, with each stair having a height equal to the voltage V_(CELL) ofthe respective battery cell C_(i). Each cell monitoring device CM_(i)uses the fact that there is only a small difference between thereference voltages of adjacent cells and that therefore the transceivers45 and 47 only have to increase or decrease the reference voltage of thereceived data by this voltage difference.

In this embodiment, the transceivers 45 and 47 comprise voltagecomparators and the messages transmitted to and from the central batterymonitoring system 3 are encoded within the transitions of a square wavesignal. FIG. 5a is a circuit diagram of a voltage comparator 61 formingpart of the transceiver 45 provided in the up-link 41 shown in FIG. 3.The limits of the comparator 61 are V_(REF) ^(i) +5V and V_(REF) ^(i)-5V, which are generated by the DC to DC converter 57. FIG. 5b is acircuit diagram of a voltage comparator 63 forming part of thetransceiver 47 provided in the down-link 43 shown in FIG. 3. As withcomparator 61, the limits of comparator 63 are V_(REF) ^(i) +5V andV_(REF) ^(i) -5V.

FIG. 5c shows part of the battery-cell voltage distribution shown inFIG. 4 and, superimposed thereon, data pulses for illustrating the wayin which data is passed along the communication link 9. The left-handside of FIG. 5c shows the ground or reference voltage V_(REF) ^(i-1) forcell C_(i-1) and shows that data pulses 65 output by cell monitoringdevice CM_(i-1), vary between V_(REF) ^(i-1) +5V and V_(REF) ^(i-1) -5V.In this embodiment, when the data is originally transmitted from thecentral battery monitoring system 3, the data pulses 65 will betransmitted from cell C_(i) ⁻¹ to cell C_(i) and will be applied to thepositive input of the comparator 61 on the up-link 41 of cell monitoringdevice CM_(i) via switch 49. As shown in FIG. 5a, the received pulsesare compared with V_(REF) ^(i) -2V (which is an approximation of thereference voltage V_(REF) ^(i-1) of the cell C_(i) ⁻¹ which generatedthe received pulses 65, since the cells are lead acid battery cellswhich provide approximately 2 volts each) and the data pulses 67 outputby comparator 61 will correspond with the received data pulses 65 butwill vary between V_(REF) ^(i) +5V and V_(REF) ^(i) -5V, as shown in themiddle of FIG. 5c. Therefore, the DC level of the square wave pulses hasbeen increased by passing it through the comparator 61.

The output data pulses 67 are transmitted to the next cell monitoringdevice CM_(i+1) via switch 51 and communications link 9. The data pulses67 output from comparator 61 are also input to the microcontroller 31via connection 53, so that the microcontroller 31 can identify whetheror not the communication from the central battery monitoring system 3 isdirected to it. If the communication is directed to it, themicrocontroller 31 processes the request, performs the necessary testsand transmits the appropriate data back to the central batterymonitoring system 3.

When data pulses 69 are transmitted to cell monitoring device CM_(i)from cell monitoring device CM_(i+1) for transmitting back to thecentral battery monitoring system 3, the received data pulses 69, whichvary between V_(REF) ^(i+1) +5V and V_(REF) ^(i+1) -5V, are applied tothe positive input of comparator 63 on the down-link 43 of cellmonitoring device CM_(i) via switch 51. As shown in FIG. 5b, thereceived pulses 69 are compared with V_(REF) ^(i) +2V (which is anapproximation of the reference voltage V_(REF) ^(i+1) of the cellC_(i+1) which generated the received pulses 69, since the cells are leadacid battery cells which provide approximately 2 volts each). As shownin FIG. 5c, this comparison results in a series of pulses 67corresponding to the received pulses 65 but which vary between V_(REF)^(i) ±5V which are transmitted to cell C_(i-1) via switch 49. Therefore,the DC level of the square wave pulses has been reduced by passing itthrough the comparator 63.

Each of the cell monitoring devices CM_(i) operate in a similar manner.However, it should be noted that the first cell monitoring device CM_(i)has the same ground or reference voltage as the central batterymonitoring system 3. Therefore, it is not necessary to use a transceiver45 in the up-link 41 of the first cell monitoring device CM_(i),although one is usually used in order to buffer the received signals andin order to standardise each of the cell monitoring devices CM_(i).Similarly, the last cell monitoring device CM_(n) will not receive datapulses from a subsequent cell monitoring device and therefore, does notneed a transceiver 47 in its down-link. However, one is provided so thatall the cell monitoring devices CM_(i) are the same, and is used forbuffering the data sent from microcontroller 31 of cell monitoringdevice CM_(n) back to the central battery monitoring device 3.

The battery monitoring system described above has the followingadvantages:

(1) There is no need for voltage isolation between the cell monitoringdevices CM_(i) or between the first cell monitoring device CM₁ and thecentral battery monitoring system 3. Therefore, each cell monitoringdevice CM_(i) will only consume a few milli-amps and only requires veryinexpensive and readily available DC to DC converters for converting thebattery cell voltage to the supply voltage needed by the microcontroller31 and the transceivers 45 and 47.

(2) Since electrical isolation is not required between the cellmonitoring devices CM_(i), there is no longer a need for relativelyexpensive voltage isolated links between the cell monitoring devices. Inthe embodiment described, each cell monitoring device CM_(i) is linkedto its neighbours by a simple wire. The cost of the battery monitoringsystem is therefore low and system installation is simplified.

(3) Continuous monitoring of all the cells C_(i) in battery 1 becomeseconomical and practical, and the user can be informed in real-time ifone or more of the battery cells C_(i) is under performing or is faulty.

(4) The internal resistance of each cell C_(i) can be determined inreal-time and without having to disconnect the cell from the battery,since the central battery monitoring system 3 is capable of measuringbattery charging and discharging current (which is the same as the cellcurrent) and can correlate it with individual cell voltages (determinedby the cell monitoring devices) in order to calculate each cell'sinternal resistance.

(5) Each cell monitoring device CM_(i) is able to measure the voltagedrop on cell to cell interconnections and indicate a faultyinterconnection condition, usually due to chemical deposits accumulatingat the cell terminals with time or because of cell malfunction.

(6) Since each cell monitoring device CM_(i) is able to measure the cellvoltage and the cell temperature, it is possible to increase theprobability of detecting a faulty cell. Therefore, the industrialbattery need only be serviced when required.

(7) Since each cell monitoring device CM_(i) can read the correspondingcell voltage, cell temperature etc. at the same time as the other cellmonitoring devices, the data produced by each cell monitoring device isless likely to be corrupted by changes in load and/or changes in ambienttemperature which occur with time, as compared with prior art systemswhich take readings from the individual cells one at a time.

A number of alternative embodiments will now be described, which operatein a similar manner to the first embodiment. Accordingly, thedescription of these alternative embodiments will be restricted tofeatures which are different to those of the first embodiment.

FIG. 6 is a schematic diagram showing, in more detail, one of the cellmonitoring devices CM_(i) shown in FIG. 1 according to a secondembodiment of the present invention. As shown, cell monitoring deviceCM_(i) comprises a microcontroller 31 for controlling the operation ofthe cell monitoring device CM_(i) and for analysing sensor data receivedfrom voltage interconnection sensor 33, cell voltage sensor 35,temperature sensor 37 and electrolyte level/PH sensor 39, which alloperate in the same manner as in the first embodiment.

In order to power the cell monitoring device CM_(i), the positiveterminal C_(i) ⁺ and the negative terminal C_(i) ⁻ of cell C_(i) areconnected to the input of a DC to DC convertor 57, which generates,relative to the ground of cell C_(i) (which equals the voltage potentialof the negative terminal C_(i) ⁻ of cell C_(i)) the voltage V_(cc) ^(i),which is used to power the microcontroller 31 and the other componentsin the device and which in this embodiment is C_(i) ⁻ +3 Volts.

Since the cell monitoring devices are connected in series by thecommunication link 9, each cell monitoring device CM_(i) is operable (i)to receive up-link communications originating from the central batterymonitoring system 3 on wire 9a of the communication link 9 for receptionby itself and/or for onward transmission to the next cell monitoringdevice CM_(i+1) ; (ii) to receive down-link communications from cellmonitoring device CM_(i+1) on wire 9b of the communication link 9 fortransmission back to the central battery monitoring system 3; and (iii)to transmit down-link communications generated by itself back to thecentral battery monitoring system 3 on wire 9b of the communication link9.

As shown in FIG. 6, in this embodiment, the microcontroller 31 receivesup-link communications originating from the central battery monitoringsystem 3 via wire 9a, potential divider 41 and comparator 43. Themicrocontroller 31 identifies whether or not the received message fromthe central battery monitoring system is for it or if it is for onwardtransmission to the next cell monitoring device CM_(i+1). If the messageis for the current cell monitoring device CM_(i), then themicrocontroller 31 decodes the message and takes the appropriate action.If the received message is for onward transmission, then themicrocontroller 31 regenerates the message and outputs it to wire 9a viaoutput block 45. In this embodiment, the messages transmitted aresquare-wave signals representing digital data. The signals are encodedfor error correction purposes and the microcontroller 31 checks forerrors in the received messages. Since the microcontroller 31regenerates the messages for transmission to the next cell monitoringdevice CM_(i+1), the timing between the transitions in the signal levelscan be resynchronised, thereby reducing any errors caused by thetransmission of the pulses along the communication link 9.

In a similar manner, down-link messages received from cell monitoringdevice CM_(i+1) on wire 9b are passed via potential divider 47 andcomparator 49 to the microcontroller 31. In this embodiment, alldown-link communications are for transmission back to the centralbattery monitoring system 3. Therefore, the micro controller 31 checksfor any errors in the received data and, if possible, corrects them. Themicroprocessor 31 then regenerates the message and outputs it to wire 9bvia output block 51 for onward transmission back to the central batterymonitoring system 3.

The way in which the messages are transmitted between the cellmonitoring devices will now be described in more detail with referenceto FIG. 7. Up-link messages originating from the central batterymonitoring system 3 which are re-transmitted by the microcontroller 31of cell monitoring device CM_(i-1) are applied on line 32 to the gateelectrode of the MOSFET Q₁ ^(i-1). The source electrode of the MOSFET Q₁^(i-1) is connected to the ground C_(i-1) ⁻ of cell monitoring deviceCM_(i-1) and the drain electrode is connected, via resistors R_(a) ^(i),R_(b) ^(i) and R_(c) ^(i-1), to V_(cc) ^(i) output by the DC/DCconverter 57 in cell monitoring device CM_(i). In operation, when themicrocontroller 31 in the cell monitoring device CM_(i-1) outputs avoltage low (representing a binary 0) on line 32, the MOSFET Q₁ ^(i-1)does not allow current to flow from the drain electrode to the sourceelectrode and therefore effectively open circuits the connection betweenV_(cc) ^(i) and C_(i-1) ⁻. Therefore, when a voltage low is applied tothe gate electrode of MOSFET Q₁ ^(i-1), a voltage of approximatelyV_(cc) ^(i) is applied on line 34 to the comparator 43, where it iscompared with reference voltage V_(ref1) ^(i). When the microprocessor31 of cell monitoring device CM_(i-1) outputs a voltage high(representing a binary 1) on line 32, the MOSFET Q₁ ^(i-1) is switchedon and current can flow from the drain electrode to the sourceelectrode. Therefore, current will flow from V_(cc) ^(i) throughresistors R_(a) ^(i), R_(b) ^(i) and R_(c) ^(i-1) through the MOSFET Q₁^(i-1) to the ground C_(i-1) ⁻ of cell monitoring device CM_(i-1). As aresult, V_(cc) ^(i) -X volts will be applied on line 34 to thecomparator 43, where it is compared with the reference voltage V_(ref1)^(i). The value of X depends upon the difference between V_(cc) ^(i) andC_(i-1) ⁻ and the values of the resistors R_(a) ^(i), R_(b) ^(i) andR_(c) ^(i-1). Provided the value of the reference voltage V_(ref1) ^(i)is between the voltage levels applied to the comparator 43 on line 34when the MOSFET Q₁ ^(i-1) is switched on and when it is switched off,the output of the comparator 43 will be a square-wave signal 36 varyingbetween the ground potential C_(i) ⁻ and V_(cc) ^(i) of cell monitoringdevice CM_(i) in synchronism with the variation of the square-wavesignal 38 applied to the gate electrode of MOSFET Q₁ ^(i-1) of cellmonitoring device CM_(i-1). Therefore, messages encoded within thevariation of the signal applied to the MOSFET Q₁ ^(i-1) in cellmonitoring device CM_(i-1) are transferred from cell monitoring deviceCM_(i-1) to cell monitoring device CM_(i).

In a similar manner, down-link messages output by the microcontroller 31in cell monitoring device CM_(i) for transmission back to the centralbattery monitoring system 3 are applied to the gate electrode of MOSFETQ₂ ^(i) on line 40. The drain electrode of MOSFET Q₂ ^(i) is raised tothe potential V_(cc) ^(i) and the source is connected, via resistorsR_(d) ^(i), R_(e) ^(i-1) and R_(f) ^(i-1), to the ground potentialC_(i-1) ⁻ of cell monitoring device CM_(i-1). In operation, when themicrocontroller 31 in the cell monitoring device CM_(i) outputs avoltage low on line 40, the MOSFET Q₂ ^(i) does not allow current toflow from the drain electrode to the source electrode and thereforeeffectively open circuits the connection between V_(cc) ^(i) and C_(i-1)⁻. Therefore, when a voltage low is applied to the gate electrode ofMOSFET Q₂ ^(i), approximately zero volts is applied on line 42 to thecomparator 49, where it is compared with reference voltage V_(ref2)^(i-1) When the microcontroller 31 of cell monitoring device CM_(i)outputs a voltage high on line 40, the MOSFET Q₂ ^(i) is switched on andcurrent flows from V_(cc) ^(i) through resistors R_(d) ^(i), R_(e)^(i-1) and R_(f) ^(i-1) to the ground C_(i-1) ⁻ of the cell monitoringdevice CM_(i-1). As a result, V_(cc) ^(i) -Y volts will be applied online 42 to the comparator 49, where it is compared with the referencevoltage V_(ref2) ^(i-1). The value of Y depends upon the differencebetween V_(cc) ^(i) and C_(i-1) ⁻ and the values of resistors R_(d)^(i), R_(e) ^(i-1) and R_(f) ^(i-1). Again, provided the value of thereference voltage V_(ref2) ^(i) ^(i-1) is between the voltage levelsapplied to the comparator 49 on line 42 when the MOSFET Q₂ ^(i) isswitched on and when it is switched off, the output of the comparator 49will be a square-wave signal 44 varying between the ground potentialC_(i-1) ⁻ and V_(cc) ^(i-1) of cell monitoring device CM_(i-1) insynchronism with the variation of the signal 46 applied to the gateelectrode of MOSFET Q₂ ^(i) of cell monitoring device CM_(i). Therefore,messages encoded within the variation of the signal applied to MOSFET Q₂^(i) are transferred from cell monitoring device CM_(i) to cellmonitoring device CM_(i-1).

The values of the resistors R_(a) ^(i) to R_(f) ^(i) in each cellmonitoring device CM_(i) are chosen in order (i) to buffer the input andoutput terminals of the cell monitoring devices CM_(i) ; (ii) to reducepower consumption of the cell monitoring devices CM_(i) ; and (iii) toprovide the necessary voltage division with respect to the difference involtage between adjacent cell monitoring devices.

As those skilled in the art will appreciate, the above technique fortransferring up-link and down-link data between cell monitoring devicesCM_(i-1) and CM_(i) will only work provided the difference betweenV_(cc) ^(i) and C_(i-1) ⁻ does not exceed the operating range of theMOSFET switches Q₁ ^(i-1) and Q₂ ^(i). In this embodiment, each batterycell C_(i) is provided with a cell monitoring device CM_(i), thedifference in operating potentials of adjacent cell monitoring devicesis approximately two volts and V_(cc) ^(i) is three volts more than theground potential of the cell monitoring device. Therefore, in thisembodiment, the difference between V_(cc) ^(i) and C_(i-1) ⁻ isapproximately five volts. MOSFET transistors Q which can operate withsuch a loading are readily available. Indeed, there are somecommercially available MOSFETs which can operate with a loading of up tosixty volts. Therefore, this technique of transmitting data betweenadjacent cell monitoring devices can be used in most practicalsituations, even in an embodiment where, for example, one cellmonitoring device is provided for every tenth battery cell C_(i).

Each of the cell monitoring devices CM_(i) operate in a similar manner.However, it should be noted, that in this embodiment, the first cellmonitoring device CM₁ has the same ground or reference voltage as thecentral battery monitoring system 3. Therefore, in this embodiment, itis not necessary to use the potential divider 41 and the comparator 43of the up-link nor the output block 51 in the down-link of the firstcell monitoring device CM₁, although these are usually provided in orderto standardise each of the cell monitoring devices CM_(i). Similarly,the last cell monitoring device CM_(n) will not transmit data to norreceive data from a subsequent cell monitoring device. Therefore, cellmonitoring device CM_(n) does not need the output block 45 in theup-link nor the potential divider 47 and the comparator 49 in thedown-link. However, these are usually provided so that all the cellmonitoring devices CM_(i) are the same.

As those skilled in the art will appreciate, in the above embodiment,solid state switches were used to, effectively, shift the DC level ofthe received signals for onward transmission. FIG. 8 illustrates afurther embodiment which uses analogue switches to transmit messagesuplink as well as downlink between adjacent cell monitoring devicesCM_(i), via a single communication wire 9 which connects the cellmonitoring devices in series. As shown in FIG. 8, each cell monitoringdevice CM_(i) has the same potential dividers 41 and 49, comparators 43and 47, and output blocks 45 and 51 as in the second embodiment. Thedifference between this embodiment and the second embodiment is that theuplink and the downlink between adjacent cell monitoring devices share acommon communication wire 9. This is achieved by connecting, atconnection 62, the output of block 51 in cell monitoring device CM_(i)to the connection between potential divider 41 in cell monitoring deviceCM_(i) and the output block 45 in cell monitoring device CM_(i-1) and byconnecting, at connection 64, the potential divider 49 in cellmonitoring device CM_(i-1) to the connection between potential divider41 in cell monitoring device CM_(i) and the output block 45 in cellmonitoring device CM_(i-1).

Since both the uplink and the downlink connection between adjacent cellmonitoring devices is via the same wire 9, communications between cellmonitoring devices can be in one direction only at any given time. Toachieve this, during a downlink communication, MOSFET Q₁ ^(i-1) isswitched off so that messages transmitted by switching the state ofMOSFET Q₂ ^(i) pass via the potential divider 49 and the comparator 47into the microcontroller (not shown) in cell monitoring device CM_(i-1).Similarly, for uplink communications from, for example, cell monitoringdevice CM_(i) to cell monitoring device CM_(i+1), the MOSFET Q₂ ^(i+1)is switched off so that the output block 51 has a high impedance anddoes not affect the operation of the potential divider 41. Further, asthose skilled in the art will appreciate, in order for the potentialdividers and output blocks to operate in the same way as in the secondembodiment, the resistances of resistors R_(a), R_(b), R_(e) and R_(f)must be relatively large compared to the resistances of resistors R_(c)and R_(d). In this embodiment, with 2 volt battery cell voltages, R_(a)=R_(b) =R_(e) =R_(f) =10K ohms, R_(d) =R_(c) =2K ohms, V_(ref1) ^(i)=V_(ref2).sup.^(i) =V_(cc) ^(i) /2 and V_(cc) ^(i) =C_(i) ⁻ +5V. Asthose skilled in the art will appreciate, the cell monitoring device ofthis embodiment can easily be adapted to operate with any battery cellvoltage, the only changes that are required are the resistor values, thereference voltage values and the maximum allowable drain to sourcevoltage of the MOSFETs.

As those skilled in the art will appreciate, in the above embodiments,data was transmitted between adjacent cell monitoring devices by varyingthe output impedance of an output block in the cell monitoring devicewhich is to transmit the message and by detecting this variation in thecell monitoring device which is to receive the message. For example,when up-link message data is to be transmitted from cell monitoringdevice CM_(i-1) to cell monitoring device CM_(i), the impedance ofoutput block 45 in cell monitoring device CM_(i-1) is varied independence upon the data to be transmitted--when a voltage low is to betransmitted, the impedance of output block 45 is made to be very high,whereas when a voltage high is to be transmitted, the impedance ofoutput block 45 is made to be relatively low. This variation of theimpedance of output block 45 is detected by the potential divider 41 andthe comparator 43 in the cell monitoring device CM_(i) which is toreceive the transmitted up-link message. As those skilled in the artwill appreciate, there are various ways of varying an output impedanceof a cell monitoring device in dependence upon the data to betransmitted and various ways of detecting that variation in the adjacentcell monitoring device. FIG. 9 shows one of these alternativeembodiments.

In the embodiment shown in FIG. 9, the output impedance of cellmonitoring device CM_(i-1) is varied in the same way as it was varied inthe second embodiment but this variation is detected in cell monitoringdevice CM₁ in a different manner. In particular, in this embodiment, acurrent detector 48 is used to detect the changes in current flowingbetween the V_(cc) ^(i) terminal in cell monitoring device CM_(i) to theground potential C_(i-1) ⁻ in cell monitoring device CM_(i-) ₁. Inoperation, when a voltage low is applied to the gate electrode of MOSFETQ₁ ^(i-1), the MOSFET is open and no current flows down line 50.However, when a voltage high is applied to the gate electrode of MOSFETQ₁ ^(i-1), the MOSFET opens and current is drawn down line 50. Thischange in current is detected by the current sensor 48. Morespecifically, each time there is a transition from a voltage high to avoltage low (or vice versa) applied to the gate electrode of MOSFET Q₁^(i-1), a voltage spike is induced in the current detector 48. Asillustrated in FIG. 9, in response to up-link message data 52 beingapplied to the MOSFET Q₁ ^(i-1), a train of voltage spikes 53 areinduced in the current detector 48 and passed to a spike detector 54which regenerates and outputs the up-link message data 52 fortransmission to the microcontroller (not shown). The resistors R₁ and R₂are provided in order to reduce the power consumed by each of the cellmonitoring devices CM_(i) and in order to buffer the input and output ofthe respective cell monitoring devices.

In the last three embodiments, MOSFET switches were used as a devicewhose impedance can be varied in dependence upon the message data to betransmitted. As those skilled in the art will appreciate, these MOSFETscan be replaced by any electronic switches (solid-state relays,electromechanical relays, J-FETs, transistors etc.) and, in embodimentswhere the up-link and the down-link messages are received andre-transmitted by a microcontroller, can be omitted altogether. This isbecause the output impedance of the output pin of the microcontrollervaries in dependence upon whether it is outputting a voltage high or avoltage low. Therefore, for example, the output pin from themicrocontroller in cell monitoring device CM_(i-1) could be directlyconnected to the potential divider 41 in cell monitoring device CM_(i).However, such an embodiment is not preferred, because themicrocontroller can be damaged by the voltage applied to the output pinfrom the adjacent cell monitoring device.

In the above embodiments, each cell monitoring device CM_(i) has amicrocontroller 31 for receiving messages from the central batterymonitoring system 3, for analysing data from various sensors and forsending data back to the central battery monitoring system 3 via thecommunication link 9. FIG. 10 schematically shows an alternative cellmonitoring device CM_(i) of an alternative embodiment which does not usea microcontroller 31.

In particular, as shown in FIG. 10, each cell monitoring device CM_(i)comprises a signal generator 71 which receives sensor signals from thecell voltage sensor 35 and the temperature sensor 37 and outputs, online 73, a signal which varies in dependence upon the received sensorsignals. The signal generator 71 may comprise a voltage controlledoscillator which outputs an alternating signal whose frequency varies independence upon an input voltage from, for example, the cell voltagesensor 35. The signal output from the signal generator 71 is applied toan output terminal 75 for transmission to the central battery monitoringsystem 3, via the communication link 9. In this embodiment, each cellmonitoring device CM_(i) only transmits signals back to the centralbattery monitoring system 3, they can not receive messages from thecentral battery monitoring system. Therefore, only a down-link isrequired to receive signals at input terminal 77, transmitted from cellmonitoring device CM_(i+1).

As in the first embodiment, each cell monitoring device CM_(i) ispowered by the cell C_(i) which it is monitoring. This is illustrated inFIG. 6 by the connections C_(i) ⁺ and C_(i) ⁻ which are connected toinput terminals 74 and 76 respectively. Since the communication link 9connects each of the cell monitoring devices CM_(i) in series in a daisychain configuration, cell monitoring device CM_(i) will receive signals,at input terminal 77, from cell monitoring device CM_(i+1). The receivedsignals are applied to a DC level shift circuit 79 which reduces the DClevel of the received signals and supplies them to the output terminal75 for transmission to the next cell monitoring device CM_(i-1) in thecommunication link 9.

In the above embodiments, the system described was a battery monitoringsystem. FIG. 11 schematically shows an embodiment which is a controlsystem for controlling the cells of an industrial battery. As shown, thecontrol system has a similar architecture to the battery monitoringsystem shown in FIG. 1, except that the central battery monitoringsystem 3 is now a central battery control system 80 and the cellmonitoring devices CM_(i) are now battery cell control devices CC_(i).As in the monitoring system of FIG. 1, the central battery controlsystem 80 communicates with each of the cell controlling devices CC_(i)via the communication link 9.

FIG. 12 schematically shows an example of one of the battery cellcontrol devices CC_(i) shown in FIG. 11. In this example, each cellcontrolling device CC_(i) is used to control the topping up of acid andwater in the respective battery cell C_(i), in response to anappropriate control signal received from the central battery controlsystem 80. As in the above embodiments, each cell control device CC_(i)is powered by the cell which it is to control, as represented by inputsC_(i) ⁺ and C_(i) ⁻ applied to input power terminals 81 and 85respectively. In this embodiment, each cell controlling device CC_(i) isarranged to receive messages from the central battery controlling system(not shown), but not to transmit messages back. Accordingly, signalsreceived at the input terminal 85 from cell controller CC_(i-1) areapplied to DC level shift circuit 87, which-increases the DC level ofthe received signals and outputs them to output terminal 89 fortransmission to the next cell controlling device CC_(i+1). Themicrocontroller 91 monitors the received signals via connection 93 andoutputs appropriate control signals to output terminals 95 and 97 whenthe received signals are directed to it. The control signals output toterminals 95 and 97 are used to control the position of valves 99 and101 respectively, so as to control the amount of water and acid to beadded to the battery cell C_(i) from the water tank 103 and the acidtank 105. The microcontroller 91 determines the amount of water and acidto add with reference to the sensor signals received from theelectrolyte level/PH sensor 39.

In the above embodiments, a central battery monitoring system or acentral battery control system was provided which monitored orcontrolled the system as a whole. FIG. 13 schematically shows a cellmonitoring and control device CM&C_(i) which can be used in a combinedbattery control and monitoring system in which there is no centralbattery monitoring and control system and in which each cell monitoringand control device CM&C_(i) communicates directly with the other cellmonitoring and control devices. As in the other embodiments, each cellmonitoring and control device CM&C_(i) is powered by the cell which itis monitoring and controlling, as represented by inputs C_(i) ⁺ andC_(i) ⁻ applied to input power terminals 115 and 117 respectively.

As shown in FIG. 13, each cell monitoring and control device CM&C_(i)comprises a microcontroller 111 which receives sensor data fromtemperature sensor 37 and which outputs control data to output terminal113 for controlling, for example, a liquid crystal display (not shown)mounted on the respective cell C_(i).

In this embodiment, the communication link comprises two wires 9a and 9band therefore, switches 49 and 51 are not required to connect theup-link and the down-link to the communication link 9. Wire 9a is usedfor passing communications up the series communication link 9 from cellmonitoring and control device CM&C_(i) to cell monitoring and controldevice CM&C_(i+1) and wire 9b is used for transmitting signals down theseries communication link 9 from cell monitoring and control deviceCM&C_(i) to cell monitoring and control device CM&C_(i-1). Accordingly,the signals received by cell monitoring and control device CM&C_(i) atinput terminal 119 are applied to DC level shift circuit 121 whichincreases the DC level of the received signals and outputs them tooutput terminal 123 for transmission to cell monitoring and controldevice CM&C_(i+1). Similarly, signals received at input terminal 125 areapplied to DC level shift circuit 127 which decreases the DC level ofthe received signals and outputs them to output terminal 129 fortransmission to cell monitoring and control device CM&C_(i-1). As shown,microcontroller 111 can receive data from and transmit data to both theup-link 9a and the down-link 9b via connections 131 and 133respectively.

Various modifications which can be made to the above describedembodiments will now be described.

In the first embodiment, the transceivers 45 and 47 used in the up-linkand the down-link within each cell monitoring device CM_(i) comprises avoltage comparator. Other types of transceivers could be used. Forexample, voltage to current and current to voltage comparators could beused. In such an embodiment, the voltage to current comparators and thecurrent to voltage comparators would be arranged alternatively along thecommunication link 9 so that a voltage to current comparator isconnected to the input of a current to voltage comparator, andvice-versa.

In the first embodiment the data transmitted between cells and betweenthe first cell and the central battery monitoring systems varies betweenV_(REF) ^(i) ±5V. The value of 5 volts was chosen for convenience sincethe normal operating voltage for the microcontroller 31 is 5 volts abovethe ground voltage for that cell. Theoretically, where the datatransmitted between cells is given by V_(REF) ^(i) ±X volts, X must begreater than half the cell voltage V_(CELL) in order for the comparatorto be able to regenerate the received data pulses at the increased ordecreased potential. Practically, since the battery cells and thecomparators are not ideal, X should be at least two and a half times thecell voltage V_(CELL).

In the first embodiment, a cell monitoring device was used to monitoreach cell of the battery. In a cheaper implementation, each cellmonitoring device CM_(i) could be used to monitor two or three seriesconnected battery cells C_(i). However, in such an embodiment, where thedata transmitted between cell monitoring device is given by V_(REF) ^(i)±X volts, X should be at least two and a half times the difference inthe reference potentials between adjacent cell monitoring devices.

In the first embodiment, the received data pulses are compared with anapproximation of the ground or reference voltage of the cell which sentthe data pulses. Alternatively, the received data pulses could simply becompared with the reference voltage of the cell monitoring device whichreceives the data pulses.

In the embodiments described, the cells are connected in series. It ispossible to connect the battery cells C_(i) in a series-parallel orladder configuration. FIG. 14 shows such an interconnection of batterycells, in which cell C_(ia) is connected in parallel with cell C_(ib)and the parallel combinations C_(ia) and C_(ib) are connected in seriesfor i=1 to n. In the configuration shown in FIG. 14, a single cellmonitoring device CM_(i) is provided for monitoring each of the batterycells and the communication link 9 connects CM_(ia) to CM_(ib) andCM_(ib) to CM_(i+1a) etc. Alternatively, a single cell monitoring devicecould be used to monitor each parallel combination of battery cellsC_(ia) and C_(ib). Additionally, more than two battery cells can beconnected in parallel.

In the above embodiments, the central battery monitoring and/or controlsystem was provided at the zero volt reference voltage end of thecommunication link 9. Alternatively, the central battery monitoringand/or control system could be connected at the high reference voltageend of the communication link 9. Alternatively still, the centralbattery monitoring and/or control system could be connected at bothends, thereby forming a circular communications path in which messageswhich are transmitted to and received from the batterymonitoring/controlling system are passed in one direction through thecell monitoring/controlling devices. Therefore, each cellmonitoring/controlling device only needs either an up-link or adown-link for increasing or decreasing the DC level of the receivedsignals, depending on whether the messages are transmitted up or downthe communication staircase.

In the above described embodiments, the communication link 9 comprisedeither one or two wires. As those skilled in the art will appreciate,the communication link 9 may comprise any number of wires along whichdata can be transmitted in parallel.

In the above embodiments, a separate central battery monitoring systemor a central battery control system was provided. In an alternativeembodiment, a combined battery monitoring and control system could beused to both monitor and control the battery.

In the above described embodiments, a single battery comprising aplurality of battery cells, is monitored and/or controlled by a centralbattery monitoring and/or controlling system. FIG. 15 shows analternative embodiment where a plurality of batteries B_(i) areprovided, and wherein each battery B_(i) is monitored by its own centralbattery monitoring system BM_(i) which communicates with a remoteoperator's terminal 151 via a data bus 153. The data bus 153 may be aproprietary data link or can be the public telephone exchange. Inoperation, each of the central battery monitoring systems BM_(i)monitors the respective battery B_(i) and reports its status back to theremote operator's terminal 151, where the condition of each of thebatteries is monitored by a human operator. A similar system could alsobe provided for controlling or for monitoring and controlling aplurality of batteries.

In the first embodiment, a cell monitoring device was used to monitoreach cell of the battery. In an alternative less expensiveimplementation, each cell monitoring device CM_(i) could be used tomonitor two or more series connected battery cells C_(i).

In the above embodiments which employ MOSFET switches, the cellsignalling devices were connected in series in a daisy-chainconfiguration, with the position of each cell signalling device in theseries communication link corresponding with the position of the cell orcells which are to power the cell signalling device in the seriesconnection of battery cells. This is not essential. The cell signallingdevices can be connected in any arbitrary series configuration relativeto the series connection of battery cells. This is because the MOSFETswitches Q₁ and Q₂ operate in the same manner irrespective of thevoltage loading applied across their drain and source electrodes.However, in this case, the values of the resistors R_(a) to R_(f) ineach cell monitoring/control device will be different and will be chosenso as to provide the necessary voltage division having regard to thedifference in voltage between adjacent cell monitoring devices in thecommunications link.

The present invention is not limited by the exemplary embodimentsdescribed above, and various other modifications and embodiments will beapparent to those skilled in the art.

What is claimed is:
 1. A signalling system for use with a plurality of series connected battery cells, comprising:first and second cell signalling devices, each to be powered by a respective one or more of said plurality of battery cells; and a communication link for connecting an output terminal of said first cell signalling device to an input terminal of said second cell signalling device; characterised in that at least one of said first and second cell signalling devices comprises a DC level shift circuit which is operable (i) to receive signals transmitted from an adjacent cell signalling device; (ii) to shift the DC level of the received signals; and (iii) to output the level shifted signals for transmission to said communication link.
 2. A signalling system according to claim 1, wherein said DC level shift circuit is operable (i) to receive signals from an adjacent cell signalling device which is to be powered by a cell having a higher ground potential than that of the receiving cell signalling device; (ii) to decrease the DC level of the received signals; and (iii) to output the level shifted signals for transmission to said communication link.
 3. A signalling system according to claim 1, wherein said DC level shift circuit is operable (i) to receive signals from an adjacent cell signalling device which is to be powered by a cell having a lower ground potential than that of the receiving cell signalling device; (ii) to increase the DC level of the received signals; and (iii) to output the level shifted signals for transmission to said communication link.
 4. A signalling system according to claim 1, wherein each cell signalling device comprises at least one sensor input terminal operable to receive a signal from a sensor, which signal is indicative of a condition of the cell or cells which are to power the cell signalling device.
 5. A signalling system according to claim 4, wherein each of said cell signalling devices comprises a sensor input terminal operable to receive a signal from an electrolyte level and/or electrolyte pH sensor, which signal is indicative of the electrolyte level and/or the electrolyte pH of the cell or cells which are to power the cell signalling device.
 6. A signalling system according to claim 4, wherein each cell signalling device comprises a sensor input terminal operable to receive a signal from a voltage sensor, which signal is indicative of the voltage of the cell or cells which are to power the cell signalling device.
 7. A signalling system according to claim 4, wherein each cell signalling device comprises a sensor input terminal which is operable to receive a signal from a temperature sensor, which signal is indicative of the temperature of the cell or cells which are to power the cell signalling device.
 8. A signalling system according to claim 4, wherein each cell signalling device comprises a sensor input terminal operable to receive a signal from a voltage interconnection sensor, which signal is indicative of the voltage drop between the cell which is to power said cell signalling device and its adjacent cells.
 9. A signalling system according to claim 1, wherein each cell signalling device comprises two of said DC level shift circuits, one of which is operable (i) to receive signals from an adjacent cell signalling device which is to be powered by a cell having a higher ground potential than that of the receiving cell signalling device; (ii) to decrease the DC level of the received signals; and (iii) to output the level shifted signals for transmission to said communication link; and the other one of which is operable (i) to receive signals from an adjacent cell signalling device which is to be powered by a cell having a lower ground potential than that of the receiving cell signalling device; (ii) to increase the DC level of the received signals; and (iii) to output the level shifted signals for transmission to said communication link.
 10. A signalling system according to claim 9, wherein said communication link comprises a single wire communication bus, and wherein said two DC level shift circuits lie on two separate data transfer paths which are connectable to said single wire communication bus by a switch.
 11. A signalling system according to claim 9, wherein said two DC level shift circuits are located on separate data transfer paths, and wherein said communication link comprises a two wire communication bus for connecting the respective data transfer paths with corresponding data transfer paths of an adjacent cell signalling device.
 12. A signalling system according to claim 1, further comprising a central battery monitoring system for monitoring the battery as a whole, and wherein each of said cell signalling devices is operable to communicate, via said communication link, with said central battery monitoring system.
 13. A signalling system according to claim 12, wherein each cell signalling device comprises:at least one sensor input terminal operable to receive a signal from a sensor, which signal is indicative of a condition of the cell or cells which are to power the cell signalling device; and a signal generator operable to generate a signal in dependence upon said sensor signal and to output said generated signal for transmission to said central battery monitoring system.
 14. A signalling system according to claim 13, wherein said central battery monitoring system is operable to poll each of said plurality of cell signalling devices in turn, and wherein upon being polled, each cell signalling device is operable to return a signal back to said central battery monitoring system via said communication link, which is indicative of said condition of the cell which is to power said cell signalling device.
 15. A signalling system according to claim 13, wherein said condition is the cell voltage and wherein said central battery monitoring system is operable to measure the battery charging and discharging current and to calculate the internal resistance of each battery cell by correlating said charging and discharging current with the cell voltages determined by the respective cell signalling devices.
 16. A signalling system according to claim 13, wherein said central battery monitoring system is operable to monitor the battery voltage, the battery temperature, the total battery current and the total level of charge.
 17. A signalling system according to claim 1, wherein each of said cell signalling devices is operable to receive a control signal from said communication link and comprises a signal generator operable to generate an actuation signal in dependence upon said received control signal and to output said generated actuation signal for controlling an actuator.
 18. A signalling system according to claim 17, further comprising a central battery control system for transmitting said control signal to said communication link.
 19. A signalling system according to claim 17, wherein each cell signalling device comprises a sensor input terminal operable to receive a signal from an electrolyte level and/or electrolyte pH sensor, which signal is indicative of the electrolyte level and/or the electrolyte pH of the cell or cells which are to power the cell signalling device, and wherein upon receiving said control signal said cell signalling device is operable to output an actuation signal in dependence upon said sensor signal for controlling the addition of water and acid to the cell in order to control its electrolyte level and/or its electrolyte pH.
 20. A signalling system according to claim 17, wherein said actuation signal is for controlling a display.
 21. A signalling system according to claim 13, wherein said signal generator comprises a microcontroller which is operable to receive communications from and to transmit communications to said communication link.
 22. A signalling system according to claim 21, wherein the microcontrollers of said signalling devices are independently addressable so that communications can be directed to a selected one or more of said cell signalling devices via said communication link.
 23. A signalling system according to claim 22, wherein the microcontrollers of said cell signalling devices are operable to communicate with each other.
 24. A signalling system according to claim 1, wherein said DC level shift circuit comprises a comparator.
 25. A signalling system according to claim 24, wherein said comparator comprises a voltage comparator.
 26. A signalling system according to claim 25, wherein the communications transmitted over said communication link comprise square wave signals, and wherein each of said comparators is arranged to compare said square wave signals with a reference signal which is an approximation of the ground potential of the adjacent cell signalling device which transmitted the received square wave signals and to output a square wave signal in dependence upon whether or not the received square wave signal is greater or less than said reference signal.
 27. A signalling system according to claim 24, wherein said comparator comprises a current comparator.
 28. A signalling system according to claim 24, wherein alternate voltage to current comparators and current to voltage comparators are used in adjacent cell signalling devices.
 29. A signalling system according to claim 1, wherein said DC level shift circuit comprises a switch and wherein said signalling devices comprises means for opening and closing said switch in dependence upon the signal to be transmitted.
 30. A signalling system according to claim 29, wherein said switch comprises a transistor.
 31. A signalling system according to claim 30, wherein a source electrode of said switch is connected to a ground potential of one of said first and second cell signalling devices, wherein a drain electrode of said switch is connected to a positive terminal of the other one of said first and second cell signalling devices, wherein said means for opening and closing said switch operates on a gate electrode of the switch in dependence upon the signal to be transmitted to the other cell signalling device and wherein said other cell signalling device comprises means for sensing the change in impedance of said transistor switch.
 32. A signalling system according to claim 31, wherein said sensing means comprises a current sensor for sensing the variation of current drawn by said switch as it is opened and closed.
 33. A signalling system according to claim 31, wherein said sensing means comprises a voltage divider connected in series with said switch for sensing the change in voltage across the switch as it is opened and closed.
 34. A signalling system according to claim 1, wherein each cell signalling device comprises a DC to DC convertor which is operable to convert the cell voltage of the cell which is to power the cell signalling device, to supply voltages and a ground voltage for powering the cell signalling device.
 35. A signalling system according to claim 1, wherein a cell signalling device is provided for each of said series connected battery cells.
 36. A signalling system according to claim 1, wherein one or more of said series connected battery cells are connected in parallel with one or more additional battery cells.
 37. A cell signalling device for use in a signalling system according to claim 1, comprising:a power input terminal connectable to the cell or cells which is or are to power said cell signalling device; and at least one DC level shift circuit which is operable (i) to receive signals transmitted from an adjacent cell signalling device; (ii) to shift the DC level of the received signals; and (iii) to output the level shifted signals for transmission to the communication link forming part of said signalling system.
 38. A signalling kit for use in a signalling system according to claim 1, comprising a plurality of cell signalling devices each of which comprises:a power input terminal connectable to the cell or cells which is or are to power said cell signalling device; and at least one DC level shift circuit which is operable (i) to receive signals transmitted from an adjacent cell signalling device; (ii) to shift the DC level of the received signals; and (iii) to output the level shifted signals for transmission to the communication link forming part of said signalling system.
 39. A signalling kit according to claim 38, further comprising a communication link for connecting said plurality of cell signalling devices in series.
 40. A signalling system according to claim 1 in combination with a plurality of series connected battery cells, wherein one or more of said battery cells are connected to a respective one of said plurality of cell signalling devices, for powering said cell signalling devices.
 41. A cell signalling device according to claim 37 in combination with a battery cell, wherein the terminals of said battery cell are connectable to said cell signalling device.
 42. A signalling method using a plurality of series connected battery cells, comprising the steps of:providing a plurality of cell signalling devices and powering them with a respective one or more of said plurality of battery cells; providing a communication link for connecting said plurality of cell signalling devices in series; receiving signals transmitted from an adjacent cell signalling device; shifting the DC level of the received signals; and outputting the level shifted signals to the communication link. 